The magnetic resonance imaging technology has been widely applied to many fields for detecting the internal structures of objects. The principles of magnetic resonance imaging are as follows: after an external magnetic field has been applied, radio frequency (RF) pulses are used to excite the protons in a tissue under examination, the protons absorb certain energy to resonate; after the RF pulse emission has been stopped, the excited protons gradually release the absorbed energy in the form of signals which are then acquired, and a scanned image of the object under examination can be obtained by processing the signals using the image reconstruction technology. In the three-dimensional MRI imaging technology, the protons in a tissue under examination are excited by having a slab taken as a unit and each slab includes several slices.
The three-dimensional turbo spin echo (3D-TSE) imaging method is a commonly used imaging method in three-dimensional MRI technology, and FIG. 1 is a schematic diagram of the 3D-TSE imaging method in the prior art. Generally, the time interval between two adjacent selective excitation pulses is referred to as the repetition time (TR), and one TR includes an acquisition window (as shown by hatched portion in the figure) and a waiting time (as shown by blank blocks in the figure).
FIG. 1 shows two TRs. As shown in FIG. 1, in the acquisition window of the first TR, a selective excitation pulse is first used to excite a current slab, and then a plurality of non-selective refocusing pulses are applied thereto, in which the angles of the refocusing pulses can be identical or different. When the angles of the refocusing pulses are identical, then it is the conventional 3D-TSE imaging technology; while when the angles of the refocusing pulses are different, the 3D-TSE imaging technology having this feature is generally referred to as the imaging technology of Sampling Perfection with Application optimized Contrast by using different flip angle Evolutions (SPACE). A phase encoding gradient (not shown in the figure) is applied after each time one refocusing pulse is applied, then a frequency encoding gradient (not shown in the figure) is applied, and one echo acquisition (that is, acquisition of scanned signals) is carried out within the duration of the frequency encoding gradient, and a plurality of echo acquisitions can constitute an echo chain for use in the subsequent image reconstruction.
Within the waiting time of the first TR, the excited protons gradually return to the state before being excited; after the waiting time is ended, the excited protons have already returned to the state before being excited, and by then the scan within the first TR is completed. Within several subsequent consecutive TRs, the current slab can be scanned continuously and repeatedly, for example, within the second TR, the above process can be repeated, and after the scan of the second TR is ended, a next slab is then scanned, and the scanning method of the next slab is identical to that of the current slab.
It can be seen that in the 3D-TSE imaging technology provided by the prior art, only one slab can be scanned within one TR, and if it needs to scan the next slab, at least it has to wait for the next TR to come, and furthermore the length of the waiting time is far longer than that of the acquisition window within the TR, therefore, the imaging efficiency of the 3D-TSE imaging technology provided in the prior art is relatively low.
A new magnetic resonance imaging method is disclosed in a Chinese patent application with the application no. 201010160442.4, and the applicant thereof being Siemens Mindit (Shenzhen) Magnetic Resonance Co., Ltd, the entire contents of which are hereby incorporated herein by reference.
As shown in FIG. 2, more than one slab can be scanned in succession within one TR, and it might be as well to assume that three adjacent slabs (a first one, slab1, a second one, slab2 and a third one, slab3) along the slice selection direction can be scanned in succession within one TR. The time period t1-t2 is the acquisition window of the first slab within the first TR, and within the time period t1-t2, one selective excitation pulse and a plurality of selective refocusing pulses are applied in succession on the first slab, and within the time period t2-t7, the protons excited in the first slab gradually return to the state before being excited; the time period t3-t4 is the acquisition window of the second slab within the first TR, and within the time period t3-t4, one selective excitation pulse and a plurality of selective refocusing pulses are applied in succession on the second slab, and within the time period t4-t9, the protons excited in the second slab gradually return to the state before being excited; the time period t5-t6 is the acquisition window of the third slab within the first TR, and within the time period t5-t6, one selective excitation pulse and a plurality of selective refocusing pulses are applied in succession on the third slab, and within the time period t6-t11, the protons excited in the third slab gradually return to the state before being excited. It can be seen that in terms of the current slab, within the time period of waiting for the excited protons therein to return to the state before being excited, the scanning of the other slabs can be accomplished by the magnetic resonance imaging equipment. According to the above method, the acquisition time of one slab can be reduced to 2/N of the time as shown in FIG. 1, wherein N is the number of slabs, for example N=3 in FIG. 2.
However, in the magnetic resonance image obtained by the above imaging process, dark lines exist at the slab boundary, and such dark lines are referred to as slab boundary artifacts (SBA). The above Chinese patent application no. 201010160442.4 has proposed to eliminate the slab boundary artifacts by use of oversampling.